Semiconductor memory devices may be classified into volatile memory devices and non-volatile memory devices. Volatile memory devices lose their stored data when their power supplies are interrupted, while non-volatile memory devices retain their stored data even when their power supplies are interrupted. Volatile memory devices may include, for example, dynamic random access memory (DRAM) and static random access memory (SRAM). Non-volatile memory devices may include, for example, programmable read only memory (PROM), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory devices (EEPROM), and flash memory.
With the recent trend toward higher performance and lower power consumption of semiconductor devices, next-generation semiconductor memory devices such as ferroelectric random access memory (FRAM), magnetic random access memory (MRAM), and phase change random access memory (PRAM) have been developed. Resistances of constituent materials for these next-generation semiconductor memory devices are variable with current or voltage. Thus, the constituent materials are characterized in that their resistances are maintained even when the supply of current or voltage is cut off.
Among the variable resistance memory devices, phase change memory devices (PRAM) using phase change materials have a high operation speed and are configured to achieve a high integration density. Therefore, PRAM devices are increasingly developed.